Endocardial cushion defects are a rare form of severe congenital heart disease. The endocardial cushion is an embryonic connective tissue involved in the formation of the interatrial septum, the interior of the ventricular septum, and the leaflets and tendon cords of the mitral and tricuspid valves. Defects may result in cardiovascular malformations, and severe defects may result in atrioventricular coexistence. Symptoms of atrial septal defects of the first foramen ovale are mainly mild shortness of breath after exercise, palpitations, or frequent respiratory tract infections. Symptoms of partial and complete atrial septal defects appear earlier, often in infancy and childhood; the disease progresses more rapidly, with early onset of marked heart enlargement and severe pulmonary congestion. In patients with a single atrial cavity type of primary foramen ovale defect, mild cyanosis occurs due to the interflow of blood from the left and right atria, creating a mixture of blood. In partial endocardial cushion defects with only mild mitral valve insufficiency, children can be operated on electively at about 5 years of age. In children with early onset of heart failure or growth restriction, surgery should be performed as early as possible. Complete endocardial cushion defects, in which heart failure and pulmonary vascular disease develop in infancy, are best operated on at 3 to 6 months of age.